The thick disks of spiral galaxies as relics from gas-rich, turbulent, clumpy disks at high redshift

TL;DR

This paper investigates the formation of thick disks in spiral galaxies, proposing that internal, turbulent, clumpy phases at high redshift are the primary mechanism, contrasting with merger-based models.

Contribution

It demonstrates that internal processes in gas-rich, turbulent disks produce thick disks with constant scale height, unlike merger models which cause flaring.

Findings

Internal processes produce non-flaring thick disks.

Mergers lead to flaring inconsistent with observations.

Gas accretion preserves thick disk structure.

Abstract

The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scattering. The bulge and thick disks of modern galaxies may form this way. Simulations of minor mergers make thick disks too, but there is an important difference. Thick disks made by internal processes have a constant scale height with galactocentric radius, but thick disks made by mergers flare. The difference arises because in the first case, perpendicular forcing and disk-gravity resistance are both proportional to the disk column density, so the resulting scale height is independent of this density. In the case of mergers, perpendicular forcing is independent of the column density and the low density regions get thicker; the resulting flaring is inconsistent with observations. Late-stage gas accretion and thin disk growth are shown to preserve the constant scale heights of thick disks formed by internal evolution. These results reinforce the idea that disk galaxies accrete most of their mass smoothly and acquire their structure by internal processes, in particular through turbulent and clumpy phases at high redshift.